As the background art of the present technical field, there is known a vehicle seat described in Japanese Unexamined Patent Application Publication No. 2014-091439 (PTL 1). PTL 1 describes a seat back frame formed in a nearly rectangular shape having four frame parts, an upper frame part, a lower frame part, an inner side frame part, and an outer side frame part, which form a skeletal outline for a seat back (see paragraph 0016 and FIG. 2). The seat back frame is a pipe frame in which the four frame parts are integrally formed by bending one pipe material. The seat back frame has round (curved or arc) bent portions (coupling portions) that couple the frame parts to each other. Two end portions of the pipe material are coupled to each other in the center of the lower frame in the width direction.
The seat back frame further includes an outer bracket fixed to the outer side frame and an inner bracket fixed to the inner side frame. In the configuration, the outer bracket holds the outer end of a reclining shaft and the inner bracket holds the inner end (the end portion on the inner side) of the reclining shaft (see paragraph 0017).